The present invention relates generally to the field of disc drives, and more particularly to an apparatus and method for providing a reliable, low resistance electrical pathway through a ferrofluidic seal between a hub and a shaft of a spindle motor used in a disc drive.
Disc drives, including magnetic disc drives, optical disc drives and magneto-optical disc drives, are widely used for storing information. A typical disc drive has one or more discs for storing information in a plurality of concentric circular tracks. This information is written to and read from the discs using read/write heads mounted on actuator arms which are moved from track to track across surfaces of the discs by an actuator mechanism. The discs are mounted on a spindle which is turned by a spindle motor to pass the surfaces of the discs under the read/write heads. The spindle motor generally includes a shaft fixed to a baseplate and a hub, to which the spindle is attached, having a sleeve into which the shaft is inserted. Permanent magnets attached to the hub interact with a stator winding on the baseplate to rotate the hub relative to the shaft One or more bearings between the hub and the shaft facilitate rotation of the hub.
The spindle motor also typically includes an exclusion seal to seal interfacial spaces between the hub and the shaft. This is necessary, because lubricating fluids or greases used in the bearings tend to give off aerosols or vaporous components that migrate or diffuse out of the spindle motor and into a disc chamber in which the discs are maintained. This vapor often transports other particles, such as material abraded from the bearings or other components of the spindle motor, into the disc chamber. These vapors and particles deposit on the read/write heads and the surfaces of the discs, causing damage to the discs and the read/write heads as they pass over the discs. Thus, the migration of these contaminants into the disc chamber must be prevented.
To prevent the migration of these contaminants into the disc chamber, the latest generation of spindle motors utilize a ferrofluidic seal between the shaft and the hub. Ferrofluidic seals are described in, for example, U.S. Pat. No. 5,473,484, which is incorporated herein by reference. A typical ferrofluidic seal consists of a ferrofluid, an axially polarized annular magnet and two magnetically permeable annular pole pieces attached to opposing faces of the magnet. The ferrofluid is conventionally composed of a suspension of magnetically permeable particles suspended in a fluid carrier. Generally, the magnet and the pole pieces are fixed to the hub and extend close to but do not touch the shaft. Magnetic flux generated by the magnet passes through the pole pieces and the shaft, which is also magnetically permeable, to magnetically hold the ferrofluid in magnetic gaps between the pole pieces and the shaft, thereby forming a seal.
One shortcoming of this design is that because the hub is separated from the shaft by grease or lubricant in the bearings and by the ferrofluid in the seal, the hub, spindle and discs build-up a considerable static electric charge while rotating. This leads to electrical arcs or sparks between the discs and the read/write heads, which are grounded, and results in the loss of information and/or permanent damage to the disc drive. This is particularly a problem for magnetic disc drives that typically use inductive or magnetoresistive heads, which are easily damaged by such an electrical discharge. Thus, a reliable, low resistance electrical pathway must be established between the spindle and electrical ground to discharge or eliminate the static electrical charge.
Several approaches have been attempted to provide a reliable, low resistance electrical pathway across the ferrofluidic seal. One approach is described in U.S. Pat. No. 4,604,229, to Raj et al. (RAJ), hereby incorporated by reference. RAJ teaches providing an electrically conducting ferrofluid, which electrically couples a rotating shaft to a housing through the pole pieces.
One problem with the approach taught in RAJ is the high resistance and often unreliable electrical connection between the pole pieces and the housing. This poor electrical connection is due to the small surface area at exterior circumferences of the pole pieces through which they contact the housing. These pole pieces typically have a thickness of less than 0.1 inches and often as little as 0.03 inches. Moreover, due to dimensions selected to facilitate the insertion of the ferrofluidic seal between the housing and the shaft, as well as manufacturing imperfections, the pole pieces generally are not in contact with the housing all the way around their circumference. In recognition of this, RAJ teaches that a snap ring, which primarily serves to hold the ferrofluidic seal in place, may be made of an electrically conducting material to increase electrical contact between the housing and one of the pole pieces (hereinafter the top pole piece). However, any increase in electrical conductivity between the top pole piece and the housing is more than offset by the teaching in RAJ of an o-ring seal between the other, lower pole piece and an inwardly projecting annular portion of the housing. The o-ring lifts the lower pole piece away from the housing, thereby reducing or eliminating contact therebetween and increasing the resistance of the electrical connection between the lower pole piece and the housing. In addition, if a grease, such as is frequently applied to o-rings, is used it migrates into interfacial spaces between the lower pole piece and the housing, further increasing the electrical resistance.
Another generally known approach for providing an electrical pathway across a ferrofluidic seal, which avoids some of the problems of the approach taught in RAJ, is described in U.S. Pat. No. 5,238,254, to Takii et al. (TAKII), hereby incorporated by reference. TAKII teaches using a conductive adhesive, such as a silver epoxy, to couple the pole pieces to the hub or housing. This approach has the additional advantage of being able to seal the lower pole piece to an inwardly projecting annular portion of the hub or housing without an o-ring, which as explained above can increase electrical resistance between the lower pole piece and the housing. However, while a significant improvement over RAJ, this approach is also not wholly satisfactory.
A fundamental problem with this approach is the increased manufacturing time and costs associated with applying the conducting adhesive, distributing it on surfaces to joined and baking the assembled pieces to cure the adhesive. Moreover, because it is necessary to spin test the spindle motor prior to final assembly, the conducting adhesive is generally applied to the upper and lower pole pieces in two separate steps. The lower pole piece is adhered to the hub in a first step prior to spin testing, and the top is adhered to the hub in a second step following a successful test. Thus, the assembled pieces must be baked twice, thereby further increasing the manufacturing time associated with this approach. Furthermore, should the spindle motor fail the spin test, the conducting adhesive generally prevents disassembly and repair of the spindle motor, thereby lower the yield of the manufacturing process. All of the above, i.e., increased manufacturing time and costs, and lowered yields, is extremely undesirable in an industry such as the disc drive industry in which competition has reduced profits to a thin margin. Finally, the use of electrically conductive adhesive is especially problematic in spindle motors used for disc drives because of the possibility of mis-applied or excess adhesive coming loose and contaminating the disc chamber or interfering with the bearings or the ferrofluidic seal itself.
Yet another generally known approach for providing an electrical pathway across a sealing member is described in U.S. Pat. No. 5,050,891, to Ishikawa (ISHIKAWA), hereby incorporated by reference. ISHIKAWA teaches providing an electrically conducting ferrofluid and an electrically conductive nonmagnetic coating or film over the surfaces of the pole pieces and magnet. The film can be chrome, gold or nickel, and is applied by plating, coating or by vapor deposition. The film need not be present over the entire surface of the sealing member. In one alternative, ISHIKAWA teaches applying a silver paste to only an internal circumference of the sealing member. Thus, it is clear that ISHIKAWA is focused solely on increasing a conducting area between the ferrofluid and the sealing member. Accordingly, a fundamental problem with the approach of ISHIKAWA is that it fails to teach a technique for increasing the electrical contact between the sealing member and a housing or hub. In fact, ISHIKAWA does not even mention such a housing or hub. It can be inferred that if the sealing member taught by ISHIKAWA is electrically connected to a hub, it is connected by one of the approaches outlined above, and therefore the resulting electrical pathway is unsuitable for the reasons given above.
Another problem with approach taught in ISHIKAWA is that applying an electrically conductive coating to the internal circumference of the sealing member decreases a radial gap separating it from the shaft or increases the magnetic gap (the separation between the pole pieces and the shaft), either of which is undesirable. The radial gap separating the sealing member from the shaft is typically less than 0.01 inch often as little as 0.004 inch. Thus, applying anything to the sealing member that reduces the radial gap increases the potential for damage to the sealing member or shaft during assembly or in operation, and hence is undesirable. Similarly, the strength of the magnetic flux that magnetically holds the ferrofluid between the pole pieces and the shaft, is inversely proportional to the size of the magnetic gap. Therefore, moving the sealing member away from the shaft to allow for the coating is likewise undesirable since it decreases the effectiveness of the seal. Finally, because of the time involved in applying the electrically conductive coating and the capital cost of equipment for doing so, the approach taught by ISHIKAWA results in increased manufacturing times and costs, which, as explained above, is extremely undesirable in the highly competitive disc drive industry. Consequently, the approach taught in ISHIKAWA is also not wholly satisfactory.
Accordingly, there is a need for a ferrofluidic seal that seals and electrically couples an outer surface of a shaft to an inner surface of a hub disposed about the shaft. It is desirable that the ferrofluidic seal provide an electrical pathway that is reliable and has low resistance. It is also desirable that a method for forming such a ferrofluidic seal not increase manufacturing time or costs for assembling a spindle motor in which the seal is used.
The present invention provides a solution to these and other problems, and offers other advantages over the prior art.
The present invention relates to a method and apparatus for sealing and electrically connecting an outer surface of a shaft to an inner surface of a hub disposed about the shaft that solves these problems.
According to one embodiment, a seal is provided having an annular magnet, with a pair of annular pole pieces coupled to opposite poles thereof. The magnet and the pole pieces are positioned between the shaft and the hub. The magnet has an interior radius that is larger than a radius of the outer surface of the shaft. The pole pieces have interior radii that are larger than the radius of the outer surface of the shaft but smaller than the interior radius of the magnet. The hub, shaft and pole pieces contain electrically conductive materials, and, in addition, the pole pieces and the shaft are also magnetically permeable. A nonmagnetic, electrically conductive ring electrically couples the pole pieces to one another and to the hub. The electrically conductive ring can be made of brass, chromium, copper, gold, nickel or alloys thereof. An electrically conductive ferrofluid is magnetically held between the pole pieces and the outer surface of the shaft to form a pair of axially separated seals, and to establish parallel electrical pathways between the hub and the shaft. The ferrofluid is magnetically held between each of the pole pieces and the outer surface of the shaft by a magnetic flux provided by the magnet and concentrated by the pole pieces.
The present invention is particularly useful for use in a spindle motor, such as used in a disc drive. A spindle motor generally has a base with a shaft coupled thereto, and a hub having an inner surface disposed about an outer surface of the shaft. An embodiment of a seal according to the present invention is positioned between the shaft and the hub to seal the outer surface of the shaft to the inner surface of the hub and to electrically couple the shaft to the hub.
In one version of this embodiment, the electrically conductive ring includes an annular disk having an interior radius larger than the interior radii of the pole pieces and an exterior radius substantially the same as exterior radii of the pole pieces and the magnet. The electrically conductive ring also includes one or more tabs projecting from the exterior radius. The annular disk is positioned concentric with and abutting one of the pole pieces, and the are folded over the magnet and the other pole piece to electrically couple the pole pieces to one another. In one preferred version, the inner surface of the hub has an annular shoulder, and the annular disk is positioned abutting the shoulder to electrically couple the pole pieces to the hub. In another version, the electrically conductive ring includes a band, having an interior radius substantially the same as the exterior radii of the pole pieces and the magnet, and circumferentially disposed about the pole pieces and the magnet. The band has a first edge and a second edge, each edge having one or more inwardly projecting tabs to electrically couple the pole pieces to one another.
In another aspect, the present invention is directed to a method of sealing and electrically coupling an outer surface of a shaft to an inner surface of a hub disposed about the shaft. In the method, a laminate is formed by fixing a pair of annular pole pieces to opposite poles of a magnet. The laminate is formed so as to have an exterior radius that is substantially the same as a radius of the inner surface of the hub, and an interior radius that is larger than a radius of the outer surface of the shaft. A nonmagnetic, electrically conductive ring is attached to the laminate so that the pole pieces are electrically coupled to one another. Then, the laminate, with the electrically conductive ring attached, is positioned between the outer surface of the shaft and the inner surface of the hub so that the electrically conductive ring electrically couples the hub to the pole pieces. An electrically conductive ferrofluid is injected between the laminate and the outer surface of the shaft to form a seal and to establish an electrical pathway therebetween. Preferably, the electrically conductive fluid is injected between each of the pole pieces and the outer surface of the shaft to form a pair of axially separated seals and to establish parallel electrical pathways therebetween. In one version, the electrically conductive ring includes an annular disk with an interior radius larger than the interior radius of the laminate and an exterior radius substantially the same as the exterior radius of the laminate. the electrically conductive ring further includes one or more tabs projecting from exterior radius of the annular disk. In this version, the electrically conductive ring can be attached to the laminate, for example, by placing the annular disk in a position concentric with and abutting one of the pole pieces, and folding the tabs over the laminate to electrically connect with the other pole piece so that the pole pieces are electrically coupled to one another. In one prefered embodiment of this version, the inner surface of the hub has an annular shoulder, and the laminate is positioned between the outer surface of the shaft and the inner surface of the hub so that the annular disk abuts the shoulder to electrically couple the hub to the pole pieces. Alternatively, the electrically conductive ring can include a band having an interior radius substantially the same as the exterior radius of the laminate and circumferentially disposed about the laminate. The band has first and second edges, each with one or more tabs projecting therefrom. The tabs are bent inward to attach the electrically conduction ring to the laminate and to electrically connect with the pole pieces so that the pole pieces are electrically coupled to one another.
In yet another aspect, the present invention is directed to a seal for sealing and electrically connecting an outer surface of a shaft to an inner surface of a hub disposed about the shaft, the seal having means for electrically coupling pole pieces to one another and to the hub to establish parallel electrical pathways between the hub and the shaft. As in the embodiments described above, the seal includes a magnet positioned between the shaft and the hub, and having poles to which the annular pole pieces are fixed. The pole pieces have interior radii that are larger than a radius of the outer surface of the shaft. An electrically conductive ferrofluid is magnetically held between each of the pole pieces and the outer surface of the shaft by a magnetic flux provided by the magnet through the pole pieces to form a pair of axially separated seals between the hub and the shaft. The means for electrically coupling the pole pieces to one another and to the hub generally involves a nonmagnetic, electrically conductive material. The nonmagnetic electrically conductive material can be aluminum, brass, copper, gold, nickel or alloys thereof. In one prefered embodiment, the means for electrically coupling the pole pieces includes an annular disk having an interior radius larger than the interior radii of the pole pieces and an exterior radius substantially the same as exterior radii of the pole pieces. The means further includes one or more tabs projecting from the exterior radius. The annular disk is positioned concentric with and abutting one of the pole pieces, and the tabs are folded over the other pole piece to electrically couple the pole pieces to one another.